In chemical batteries such as lithium ion secondary batteries, a separator is interposed between positive and negative electrodes in order to electronically insulate the electrodes and to retain an electrolyte. Currently, lithium ion secondary batteries typically utilize a separator comprising a microporous thin film made of polyolefin resin such as polyethylene.
Meanwhile, in order to enhance high rate and cycle life characteristics, proposed has been a structure in which a porous film layer is formed on at least one surface of at least one of positive and negative electrodes, instead of a conventional structure in which a microporous thin film is interposed between positive and negative electrodes (see specifications of Japanese Laid-Open Patent Publications Nos. Hei 10-106530 and Hei 7-220759, and U.S. Pat. Nos. 5,882,721 and 5,948,464). Currently known porous film layer comprises a binder made of resin material and a filler made of solid particles.
The formation of a porous film layer on at least one surface of at least one of positive and negative electrodes has such advantages as the simplification of a battery structure and that of the production process because it does not further require a separator comprising a microporous thin film. Additionally, a separator comprising a microporous thin film could be misaligned from its correct position, but such defect does not occur in the case where a porous film layer is formed on at least one surface of an electrode plate, and therefore short-circuiting is prevented.
Nevertheless, the binder is usually made of resin material, namely, polyvinylidene fluoride, a vinylidene fluoride-hexafluoropropylene copolymer, an ethylene-propylenediene monomer copolymer or a rubber-like resin. All of these resin materials do not have sufficient adherence. Therefore, in order to adhere a porous film layer onto the surface of an electrode plate, the porous film layer is required to have a relatively large amount of binder. When a relatively large amount of binder is contained in the porous film layer, the flexibility of the porous film layer becomes insufficient and thus the porous film layer is more likely to be damaged. At the same time, problems arise that the degradation of the binder greatly affects battery life and that high rate characteristics of the battery is impaired.